The present invention relates to a negative pressure boosting device used as a brake booster or the like and, more particularly, to a negative pressure boosting device capable of shortening the stroke of an input shaft thereof in the initial stage of its normal operation and capable of providing larger output in the event of emergency braking operation as compared to the normal operation.
Conventionally, a negative pressure boosting device utilizing negative pressure is used as a brake booster in an automobile such as a passenger car. Upon depression of the brake pedal for normal braking operation, an input shaft is moved forward to move a valve plunger, connecting the input shaft, forward. Accordingly, a valve element of a control valve arranged in the valve body is seated on a vacuum valve seat similarly formed in the valve body so as to close a vacuum valve, while an atmospheric valve seat formed in the valve plunger is spaced apart from the valve element of the control valve so as to open an atmospheric valve. Accordingly, a variable pressure chamber into which a negative pressure is introduced in the inoperative state is isolated from a constant pressure chamber into which the negative pressure is always introduced. At the same time, the variable pressure chamber is in communication with the atmosphere. Therefore, the atmosphere is introduced into the variable pressure chamber through the open atmospheric valve, resulting in a pressure difference between the variable pressure chamber and the constant pressure chamber. Because of the pressure difference, the power piston is moved forward so that the valve body and an output shaft are moved forward. In this manner, the negative pressure boosting device boosts the input (i.e. pedaling force) of the input shaft in accordance with a predetermined servo ratio to output. The output of the negative pressure boosting device moves a piston of a master cylinder, whereby the master cylinder produces a master cylinder pressure. With this master cylinder pressure, wheel cylinders are actuated, thereby operating the service braking.
Generally, the negative pressure boosting device has a jumping (JP) characteristic. That is, as shown in FIG. 6, no reaction is transmitted from the output shaft to the input shaft when the input is small, while reaction is transmitted to the input shaft via a reaction mechanism to substantially generate a predetermined output when the input is somewhat large.
As the brake pedal is released so that the input shaft is moved backward, the atmospheric valve seat abuts on the valve element of the control valve to close the atmospheric valve and, at the same time, the valve element is spaced apart from the vacuum valve seat to open the vacuum valve, whereby the variable chamber is isolated from the atmosphere and is in communication with the constant pressure chamber. Then, the atmosphere introduced in the variable pressure chamber is discharged to a negative pressure source through the open vacuum valve and the constant pressure chamber so that the negative pressure is introduced into the variable pressure chamber, thereby canceling the pressure difference between the variable pressure chamber and the constant pressure chamber. As a result of this, the power piston is moved backward so that the valve body and output shaft are moved backward to their inoperative positions, whereby the output of the negative pressure boosting device dies out. Therefore, the piston of the master cylinder is moved backward to its inoperative position so that the master cylinder pressure dies out, thereby canceling the service braking.
By the way, in a brake system including the master cylinder and the wheel cylinders, a predetermined amount of brake fluid should be supplied from the master cylinder to the wheel cylinders until the wheel cylinders actually generate braking force after the master cylinder is actuated to start the supply of brake fluid to the wheel cylinders. The travel of the piston of the master cylinder until the wheel cylinders actually generate braking force is called loss stroke. For the loss stroke, the piston of the master cylinder must have longer stroke. This makes the output shaft and the input shaft of the negative pressure boosting device longer and, as a result, makes the pedal stroke longer.
Accordingly, Japanese Patent Unexamined Publication No. H05-193486 discloses a negative pressure boosting device which can shorten the stroke of an input shaft of the negative pressure boosting device, i.e. shorten the pedal stroke, even with such a loss stroke in the brake system.
The negative pressure boosting device disclosed in the publication is provided with a sleeve having a valve seat of a vacuum valve (hereinafter, referred to as “vacuum valve seat”) at a rear portion thereof and slidably fitted to the inner periphery of the valve body. The sleeve is always urged by a spring in a direction of opening the vacuum valve. When the negative pressure boosting device is not operated, the rearmost position of the sleeve is defined by a key member restricting the backward movement of the valve body. As the input shaft is moved forward by the depression of a brake pedal, the vacuum valve is closed and an atmospheric valve is opened in the same manner as mentioned above so that the atmosphere is introduced into a variable chamber to move a power piston, the valve body, and an output shaft forward. Accordingly, the negative pressure boosting device outputs.
In the initial stage of the operation of the negative pressure boosting device, until the key member abuts on a stopper formed in the valve body and starts to move forward together with the valve body, the sleeve is held at its initial position by the spring and the vacuum valve and the atmospheric valve are held to balance with each other so that only the valve body moves relative to the input shaft. Therefore, the stroke of the valve body or the output shaft should be longer than the stroke of the input shaft. In other words, the device disclosed in the publication has a shorter stroke of the input shaft, i.e. a shorter pedal stroke, compared to the conventional negative pressure boosting device for the same stroke of the output shaft.
On the other hand, in a brake system, there is a case, such as for emergency braking, that it is necessary to produce desired large braking force more rapidly than that of the service braking operation immediately after the depression of the brake pedal. Therefore, it has been traditionally desirable to employ a brake assist (hereinafter, referred to as “BA”) control system in a brake system. By employing the BA control system, large braking force can be rapidly produced even with small pedaling force. For improving the brake boosting control, it is preferable to not only shorten the pedal stroke as described above but also perform the BA control.
To shorten the stroke of the input shaft in the initial stage of the service braking operation and to perform the BA control in the event of the emergency braking operation, there is an idea of modifying the negative pressure boosting device disclosed in the aforesaid publication to be capable of outputting desired large braking force more rapidly than that of the service braking, for example, in the event of emergency braking.
However, it is difficult to output desired large braking force more rapidly than that of the service braking operation, in the event of emergency braking, that is, it is difficult to exhibit the BA control in the negative pressure boosting device because the output must be the same for the same input applied to the input shaft even in the event of emergency braking operation.